Modulator



L. F. MAYLE May 25, 1948.

MODULAITOR Fi'led Nov. 19, 1945 INVENTOR LOUIS E MAYLE ATTONY Patented May 25, 1948 s'rr OFFICE 2,442,305 MODULATOR Application November 19, 1945, Serial No. 629,581

3 Claims.

This invention relates to radio signaling systems and particularly to means employed in such systems for supplying heating current to the cathodes of certain of the tubes used therein.

Most of the vacuum tubes used in signaling and other types of circuits at present are provided with thermionic cathodes which are heated indirectly by specially provided heater elements. It is customary to provide a separate circuit for supplying energy to the heating elements of these tubes. Also, by reason of the necessary proX- imity of the heating elements to the cathode electrodes associated therewith, it ordinarily is not practical to provide more than a normal insulation between the cathode and its heater; consequently, it is necessary to insure that the heater voltage does not diifer too greatly from the operating voltage of the cathode. Where it is impossible to operate the cathode voltage of a tube at substantially the same potential as the source of heating current, it is necessary to employ some expedient such as an isolating transformer.

Another condition which exists usually in a vacuum tube circuit is that the space current for one or more of the tubes is suppliedto the anode of the tube through a voltage-dropping or load resistor. In such a case, there is a heat loss in a resistor of this character. In order to effect the most eificient operation of any such device, it of course is necessary to maintain all losses, such as the heat loss in a load resistor, at a minimum.

It frequently happens in a circuit including vacuum tubes such as in television transmitting apparatus, for example, that the cathode of a tube is required to operate at a potential relative to ground which is approximately of the same value as the anode potential of another tube. One such television system is disclosed in a copending application of Louis F. Mayle, Serial No. 518,534, filed January 17, 1944, and entitled Pulse keying circuit. In that case space current for a power amplifier is provided through a diode. The cathode of the diode therefore is substantially at the same potential as the anode of one of the vacuum tubes included in the power amplifier. The diode is used in conjunction with a condenser and a source of impulses for the purpose of periodically increasing the anode potential of the power amplifier in order to effect the modulation of synchronizing impulses upon the carrier wave output current of the amplifier.

The present invention is an improvement on the general circuit arrangement of the copend ing Mayle application. It is customary to excite a power amplifier at a carrier wave frequency by coupling it to an oscillator or other source of carrier current. Such an oscillator usually includes one or more vacuum tubes to which it is necessary to supply space current.

It, therefore, is an object of the present invention to provide a novel circuit arrangement for deriving heating energy for an indirectly heated cathode of a thermionic device whereby to minimize heat losses in the system and thereby to increase the operating efficiency thereof.

In accordance with this invention, space current for an electronic device is supplied through the heater element for the cathode of a thermionic device. In general, there is provided a thermionic device having a cathode arranged to be activated by an indirect heating element, said cathode being operated at approximately the same potential relative to ground as the anode of a first electronic device. Space current for the electronic device is derived from a source of unidirectional current and is supplied to the anode circuit of this device through a load circuit which includes the heater element of the thermionic device. In this manner a portion at least of the space current for the electronic .device is utilized to energize the heater element of the thermionic device. Inasmuch as the cathode of the thermionic device and the anode of the electronic device are operated at approximately the same potential, the normal insulation provided between the heater element and the cathode of the thermionic device is entirely adequate.

More specifically, in accordance with a particular television transmitter circuit embodying the invention, the electronic device is a diode connected in series with a circuit supplying space current to the anode of a second electronic device which is included in a power amplifier. The first electronic device is employed as a part at least of an oscillator which is used to excite the power amplifier at a radio frequency.

For a better understanding of the invention, together with other and further objects thereof, reference is made to the following description, taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

The single figure of the accompanying drawing is a schematic circuit diagram of a portion of a television transmitter apparatus embodying the invention. I

Having reference now to the drawing, there is shown diagrammatically a power amplifier l consisting at least of one vacuum tube 2. There also is provided a source of radio frequency energy such as an oscillator 3 including at least one vacuum tube 4. It is to be understood that the vacuum tubes 2 and 4 illustrated herein are not intended to represent completely operative structures. They are intended merely to illustrate in a general manner the character of the circuits in which other apparatus, shown in greater detail herein, is connected for operation in accordance with this invention. The power amplifier I and the oscillator 3 may in fact be any desired apparatus to meet the particular requirements of a television transmitter. symbolically the output circuit of the oscillator 3 is shown coupled to the grid circuit of the power amplifier tube 2 by a resistor 5. This coupling will be understood to provide the necessary excitation of the power amplifier at the frequency generated by the oscillator. source 6 connected to the terminals of the coupling resistor for the purpose of effecting an amplitude modulation of the radio frequency output energy of the power amplifier. In the present illustrative embodiment of the invention, it is assumed that this signal source is a generator of television video signals.

Space current for the power amplifier tube 2 is derived from a source of unidirectional current such as a battery .1, the negative terminal of which is connected to the grounded cathode of the tube 2. The positive terminal of the battery '1 is connected through a double diode tube -8 to theanode of the amplifier tube 2. In one form of'the invention, a diode which has been found to give satisfactory results is an .RCA type 117Z6GT. This tube is a double diode and for the present use, the two anodes 9 are connected together and also the two cathodes H are similarly connected. The diode :8 is provided with a pair of heater elements I2 connected together inside of the tube and arranged to effect thermionic electron emission from the cathodes II when energized by suitable heating current.

Space current for the oscillator tube 4 also is derived from the battery .1, the positive terminal of which is connected to a voltage-dropping impedance device or load circuit. The other terminal of the load circuit is connected to the anode of the oscillator tube. The :load circuit comprises the series connection of the two heater elements 1:2 of the diode :8 and a resistor 13 .connected in parallel therewith. The :parallel circuitarrangement of the heater elements I? and the resistor [3 are further connected in series with another resistor :14.

A condenser l5 hasone of its plates connected to the cathodes ll of the diode 3 and also to the anode of =the=power amplifier :tube .2. The other plate of the condenser 15 is connected .toa source of positive impulses 16. In effect, this :impulse source is a periodically operatedswitch symbolized here by the switch H. The details of the impulse source form no part of the present .invention and consequently have .not been :specifically illustrated herein. One such 'source :of impulses which may be usedtoadvantage ina system of this type is disclosed fully in the .copending Mayle application referred to hereinbefore.

Referring now to the operation of the illustrativeembodiment of the invention, assume. that the function of the apparatus disclosedrisitozamplitude modulate a carrier frequency waveawith television video signals and also with system-scontrol signals such as blanking and/.or :synchronizing impulses. The power amplifier :1 :is 'ex- There also may be provided a signal I cited at the desired radio frequency by means of the oscillator 3. Amplitude modulation of the carrier wave developedby the power amplifier to represent video signals is effected by means of the signal source 6 coupled to the input circuit of the power amplifier. During this portion of each operating cycle, space current is supplied to the power amplifier tube 2 from the battery 1 through the space discharge path of the diode 8. The diode is conditioned for operation by means of the described connection of the heater elements [2 in the circuit through which space current is supplied to the oscillator tube 4. During this portion of the operating cycle, the impulse source lfi is assumed to be in such condition to effectively connect one plate of the condenser 15 to ground as indicated by the illustrated position of the switch H. At this time the diode 8, being in a conducting state, ofiers only a relatively small impedance to the flow of space current from the battery 1 to the anode of the power amplifier tube 2. Therefore the other plate of the condenser 15, which is connected to the cathodes I I of the diode ,8, is maintained substantially at the full positive potential'of the battery 1.-

In the interval between scanning periods of the television apparatus by means of which the video signals are developed, the impulse source 1 6 functions as described in the copending Mayle application to effectively move the switch I! into engagement with its upper contact. The full voltage of the batter 1 thereby is momentarily impressed upon the normally grounded plate .of the condenser 1-5. The effect of this operation is .to impress upon the cathodes ll of the diode .8 and upon the anode of the power amplifier tube 2 an impulsive voltage of approximately twice the magnitude of the voltage of the battery I. The voltage of the cathodes ,ll of the diode thereby are raised appreciably-above the ,voltage of thea-nodes 9 and consequently render this tube nonconducting. In like manner, the anode voltage :of the power amplifier tube 2 is approfimate- 1y doubled over its normal operating value. The result is that the carrier wave .developed by the power amplifier is modulated in amplitude in accordance with the impulsive voltage impressed thereon by means of the impulse source lfiand associated apparatus to represent .one .of the .sys- .tem control signals.

As in more conventional apparatus, the voltage developed at the anode of ,the'oscillator tube .4 is a function of .the impedance'of the load .circuit vconnectedbetweenthe anode of this tube and the battery Fl. The current which is :caused to travrerse this =load Joircuit in order to develop :the .necessary voltageat the anode of the oscillator tube generates heat in the load impedance device and is dissipated thereby. The heat which is dissi- -pated "by the load "impedance device represents an energy loss which-mustbe taken into account .in calculating-the overall efiiciency of the-appara- 13118. .By means of the instant inventiomhowever,

some of the heat :energy developed in the load .circuit .of the oscillator is made .to perform the useful function of activating .the ,cathodes .H of the 'diode .8. This .portion of the heat energy, (therefore, does notfoonstitute .a loss. If other :more --conventional arrangements were employed for activatingrthe diode cathodes, additionaLheat energy would have to be supplied --by .the battery .1 ,orother-gsource;of -energy andall :of the heat energydeveloped in the load circuit of the 05.611- lator would.-serve no-.useful purpose and .would represent a total loss. Therefore, by means of a circuit arrangement, in accordance with this invention it is possible to increase the overall operating efficiency of a device of this character.

It is to be understood that the described embodiment of the invention is merely by way of illustration. The thermionic device, of which the heater element is employed in accordance with the invention, need not be a diode as disclosed specifically herein but instead may be practically any other type of vacuum tube including those in which the space current is controlled by one or more grids. The thermionic device, such as the diode 8, may also be employed for purposes other than that of supplying space current to an electronic device such as the power amplifier tube 2 described herein. Similarly, the heater element of such a thermionic device is not necessarily required to be connected to the anode of an electronic device in order to be traversed by space current flowing in the output circuit of the electronic device. For example, in the proper circumstances, it is considered to be within the scope of the instant invention to connect the heater element of a thermionic device to the cathode of an electronic device in the case where the electronic device is connected in a well known manner as a cathode follower wherein the major portion of the load impedance is connected to the cathode. The only requirement, in order to realize the advantages of the arrangement according to this invention, is that the potential of the tube electrode, to which the heater element of the thermionic device is connected, be operated at a potential which does not differ from the operating potential of the indirectly heated cathode by more than the voltage normally existing between a cathode and its associated heater. In such a case, the normal insulation provided between the cathode and its heater will be adequate.

While there has been described what, at present, is considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it, therefore, is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In a signaling system, a power amplifier having a vacuum tube including an anode and a cathode, an oscillator having a vacuum tube including an anode and a cathode and being coupled to said power amplifier for excitation thereof at a carrier frequency, a source of signals coupled to said power amplifier for modulating said carrier, a source of unidirectional current having a first terminal connected to the cathodes of said power amplifier and oscillator vacuum tubes, an indirectly heated diode having an anode connected to a second terminal of said unidirectional current source and a cathode connected to the anode of said power amplifier tube to conduct space current to said power amplifier tube, a heater element for the cathode of said diode connected between the second terminal of said unidirectional current source and the anode of said oscillator tube for conducting space current to said oscillator tube, a condenser connected to the cathode of said diode and to the anode of said power amplifier tube, and a source of positive impulses to periodically increase the anode potential of said power amplifier tube and thereby modulate said carrier by said impulses.

2. In a signaling system, a power amplifier having a vacuum tube including an anode, an oscillator having a vacuum tube including an anode and being coupled to said power amplifier for excitation thereof at a carrier frequency, a source of signals coupled to said power amplifier for modulating said carrier, a source of unidirectional current having one of its terminals grounded, an indirectly heated diode having an anode connected to the ungrounded terminals of said unidirectional current source and a cathode connected to the anode of said power amplifier tube to conduct space current to said power amplifier tube, a series arrangement of a load impedance device and a heater element for the cathode of said diode connected between said unidirectional current source and the anode of said oscillator tube for conducting space current to said oscillator tube, said series arrangement being connected to the ungrounded terminal of said current source, a condenser having one terminal connected to the cathode of said diode and to the anode of said power amplifier tube, and a switching device connected to the other terminal of said condenser for periodically switching said other condenser terminal from one to the other terminal of said unidirectional current source, whereby to periodically increase the anode potential of said power amplifier tube and thereby effect an impulse modulation of said carrier.

3. In a signaling system, a power amplifier having a vacuum tube including an anode, an oscillator having a vacuum tube includng an anode and being coupled to said power amplifier for excitation thereof at a carrier frequency, a source of signals coupled to said power amplifier for modulating said carrier, a source of unidirectional current having its negative terminal grounded, an

indirectly heated diode having an anode con-V nected to the positive terminal of said unidirectional current source and a cathode connected to the anode of said power amplifier tube to conduct space current to said power amplifier tube, a series arrangement of a load resistor and a heater element for the cathode of said diode connected between said unidirectional current source and the anode of said oscillator tube for conducting space current to said oscillator tube, said heater element being connected to the positive terminal of said current source, a shunt resistor connected to the terminals of said heater element, a twoplate condenser having one plate connected to the cathode of said diode and to the anode of said power amplifier tube, and a source of impulses including a switching device connected to the other plate of said condenser for periodically switching said other plate from one to the other terminal of said unidirectional current source, whereby to periodically increase the anode potential of said power amplifier tube and thereby modulate said carrier by said impulses.

LOUIS F. MAYLE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

